The deployment of two or more spacecraft connected by variable length tethers makes possible many new options for space operation. One option is generation of electric power in space. Studies have shown that a tether towed through the charged plasma medium of space (ionosphere) to cut the geomagnetic field between two spacecraft in low orbits can provide an efficient source of electric power. In these studies, the spacecraft and tether act as a generator of very large dimensions with the current return path being through the plasma medium. These studies have shown that a 100 kilowatt (KW) power generator based on a 100 kilometer (km) electrodynamic tether can be both feasible and practical, using currently available technology.
In all of these considerations for power generation applications, the tether electric circuit is closed through the surrounding plasma medium (ionosphere), and the impedence depends on the actual condition of the plasma medium where the generator is operating. In some cases this could be favorable, for example, a 15 gauge wire with a resistance of 10 Ohms/km would allow a useable current of 10 amperes (A) to be drawn at a power level of 2.4 KW/km. The current limit depends specifically on the condition of the ionospheric plasma, the collection area that is in contact with this plasma, and the ability of the system to eject charges to the plasma environment. It is not subject to the day-night cycle limitations of solar cells. The collection area may be increased by the use of multiple tethers or an increased end body cross section for a single tether. However, to close the circuit path requires both the collection of electrons at the tethered satellite and also the emission of electrons from the base station or shuttle orbiter. The limitations on this process are yet to be determined, but to generate large currents will probably require the application of advanced cathode and electron acceleration technology. Thus, some basic research is necessary before the limits of current collection and emission mechanisms are established.
The generation of power also tends to cause an orbiter to lose altitude as kinetic energy is converted into electrical energy, and fuel is used by rockets to regain the orbiter altitude. That is, a tether generation system is reversible, kinetic energy of the orbital motion can be used to generate electrical energy or, in reverse, electrical energy can be used to increase the orbiter's altitude. The kinetic energy can be supplied either by the expenditure of chemical fuel, which would need to be resupplied, or by sufficient solar array power.
Tether power generation systems can provide a wide range of voltages and currents. For example, tethers of 200 km producing 200 amperes, for a power generation capability of 8 megawatts, or tethers as long as 500 km that may produce up to 20 megawatts are reasonable. To realize these potentials, however the current loop must not depend on the variable ionospheric plasma.